I see the D5100 does not have an ISO button - very strange for something so commonly used to not have its own button.

I have read that you can make the function button work as the ISO button.

My question is this: is making the function button the ISO button a temporary workaround that keeps getting reset and thus keeps requiring me to set it as suc,h or once I set it to function as the ISO will it stay that way until I change it again?

Also, what fucntion will be lost by making the function button the ISO control?

That's kind of an casual way to use that particular setting. First of all, ISO 6400 can produce quite a lot of noise, thereby reducing image quality, and even if you use the cameras built-in noise reduction, that function will also decrease detail, which also reduces image quality. Second, Auto ISO sensitivity control only increases the ISO sensitivity when the camera can't obtain a correct exposure at the (minimum) ISO sensitivityand the Minimum shutter speed. You should select the setting for the Minimum shutter speed based on the focal length of the lens you're using. If you're using a long focal length lens, you should set a faster shutter speed as the minimum, but that could cause Auto ISO to an unnecessarily high ISO when using a lens with a shorter focal length. And if you're using a zoom lens, you should monitor that setting as your situation changes, especially if you're using a lens with a significant zoom range.

I believe that manual control of ISO generally produces better images than any auto ISO system.
I only use the auto ISO if I am not sure what to do.
Although the camera does seem to do a great job at the high ISO settings (from the images I have seen), surely the pictures would look better at a much lower ISO (situation permitting).